Knowledge Bank

A simple theory of the C-O stretching force constants in compounds containing a metal carbonyl group is obtained based upon molecular orbital considerations. An index to the nature of the bonding is furnished by the ratio $PI=kco0-kcokco0-kco\ast$ where $kco0$ is approximated by the ground state force constant in carbon monoxide and $kco\ast$ by that of the excited C-O molecule in the $\sigma 2\pi 4\pi \ast$ configuration. Here $kco$ is the force constant observed for the metal carbonyl derivative. This index provides a first estimate of the number of electrons in the anti-bonding $\pi$ orbitals of a C-O group. The index has been calculated for representative molecules with metal carbonyl groups and the results are discussed.